Geopuzzle #16

This weeks challenge comes via fellow Scibling Martin of Aardvarchaeology:

I’ve noticed something funny about the map of North America. The Great Lakes in the US form the end of a straight line of huge lakes extending north-west across Canada. Could you please tell me how that line of lakes formed?

It looks like he has a point:


Now, this is one of those situations where I can make an educated guess about the answer, but I’m neither sure if it’s right or if I’m grasping the full story. If I’m even approaching a clue, this (right click to download) or this may prove helpful. Or not. Either way, get guessing.

Categories: geology, geopuzzling

Comments (13)

  1. Bob O'H says:

    Is the first link meant to be a page of xml? Firefox doesn’t know what to do with it, poor thing.
    As for the puzzle, my first guess would be that it’s something glacial (‘cos everything to do with lakes in the north is, isn’t it?) – did the ice sheets stop there at some point, and there’s a terminal moraine south of that? Finland does something similar, and I guess Sweden does too – possibly near where Martin lives. Denmark is just the bits of Sweden and Norway that were scoured off, and dumped into the sea.

  2. Chris Rowan says:

    It’s a Google Earth .kml file. If you have GE installed, it should run load it up automatically…

  3. NJ says:

    Looks to me like the boundary between the precambrian cratonic rocks and the paleozoic (and younger) stable platform rocks.

  4. Chris Rowan says:

    I’d say you’re right – but that doesn’t fully explain why there should be lakes there…

  5. Silver Fox says:

    I just get html with IE7, also, even w/ Google Earth installed.
    Is the line really linear, or curved? It is the outer ring of a very old impact event?

  6. Eamon Knight says:

    *ahem* Four of the five Great Lakes are half-Canadian ;-).
    NJ is roughly correct. The “line” approximately follows the southern margin of the Canadian Shield (pink area on this map). The lower Great Lakes themselves are in Paleozoic terrain. I bet Harold Asmis will be along presently with the details ;-).

  7. ScienceWoman says:

    KML link doesn’t work for me either. But Bob O’H, the LGM glacial limit is significantly farther south than those lakes. I don’t know much about the Canadian Lakes, but unlike the more southerly Great Lakes which are attributed to glacial scour, Lake Superior has been a basin since the failed North American mid-continent rift ~1.2 billion years ago (1200 Ma).

  8. Harold Asmis says:

    That’s very neat question, and one I’ve never seen before. I should’ve had it on my blog! Each lake has it’s own unique geological reason for being there, but the main reason is that it was scooped out by the glaciers, and all potential great lakes below that line have been filled in, by the great piles of dirt below that line.
    Above the line, we have pure Archean or Precambrian rock. As well, the main centre of glaciation was at Hudson Bay, and this can be seen by the isostatic rebound patterns.
    I might get some references on my blog.

  9. Chris Rowan says:

    Re: the KML file. It seems this might be a server configuration thing, so you’ll have to right click on it and save as (select “all files” to stop it being saved as a text file), then load it up in Google Earth.
    Alternatively, just got to OneGeology and get the Canadian basement geology layer (which is what it is) yourself.

  10. KC says:

    My understanding is that there isn’t one single cause. The Pembina escarpment in Manitoba following that general trend is loose dividing line between Precambrian shield and Phanerozoic stratigraphy south west toward the Williston Basin in Saskatchewan along the US/Canada border.
    One partial interpretation may be of glacial action eroding the sedimentary bedrock and some Quaternary clues points to this direction of movement. That said, a lot of Ordovician limestonee still exists near Lake Winnipeg. Another is the weight of the Laurentide icesheet depressing the crust, as Lake Winnipeg is slowly rebounding and may one day be completely gone. How that relates to the Great Lakes, or Great Slave/Bear Lakes, I cannot say.
    Many of Manitoba’s smaller lakes in the Precambrian shield are actually associated with ultramafic bodies and greenstone belts. A lot of exploration goes on in the Thompson Nickel Belt over lakes situated above peridotite, which as you may well known, is more easily eroded than the basement gneiss. However, as the TNB runs perpendicular to this trend and is relatively small geographically as a suture zone, this would only account for some of the lakes seen.

  11. WKM says:

    Harold has some good info at his site, and he emphasizes that there is no one simple answer. Glaciation has been a hugely important geological process in Canada, and only small parts of the country have not been glaciated. That was a major reason why it took a while before we had any diamond mines here. Glacial Map of Canada (1253A) published in Geology and Economic Mineral of Canada (GSC, 1968) is interesting for seeing ice movements (don’t know for sure, but there could be an updated version of this). There are some very significant linear/tectonic features that can be important for mineral exploration. KC points out the TNB in Manitoba; there is also the northeast arm of Great Slave Lake. Also the Destor-Porcupine Fault (gold) in Ontario-Quebec. Although these are small features compared to the one in question here, they are extremely large when considered in the context of minerals exploration (actually finding a mineral deposit is a needle in haystack proposition). Along this line, when I was working in Ontario in the late ’70s, a prospector/promoter (emphasis on promoter) brought us a proposal to do some work based on his “research” (I use the word in the loosest possible sense). He had drawn a line across Proterozoic and Archean boundaries and in his mind had linked various ore deposits of drastically different types, ages and origins to one event. Wonderful stuff, eh? I did an internal company report at the time, detailing some well established lineament-tectonic zones and also some of the imagined ones – and there were some bad ones. The bottom line as always is to check things out.

  12. The Pembina escarpment in Manitoba following that general trend is loose dividing line between Precambrian shield and Phanerozoic stratigraphy south west toward the Williston Basin in Saskatchewan along the US/Canada border.

  13. Bea Gee says:

    long story short- glacial path, geology, climate